Since the discovery of the high-transition-temperature superconductors (HTS), researchers have searched for a method to fabricate superconducting tunnel junctions from these materials for both superconducting electronics operating at the practical temperature of liquid nitrogen (˜77 K) and for fundamental measurements essential for testing and guiding theories of these remarkable superconductors.
There are a variety of scientific and technological reasons why it would be significant to be able to fabricate large numbers of reproducible, high-quality Josephson junctions from high-transition-temperature superconductors (HTS). Tunnel junctions would allow the spectroscopic study of HTS materials in the a-b plane allowing access to direct symmetry and excitation spectroscopies. Large scale circuits with millions of Josephson junctions on a chip could open new applications in high-performance computing, high-frequency sensors, and magnetometry that have been previously unsuccessful. A challenge associated with reaching these goals is that HTS materials are difficult to process and the superconducting coherence length is both short and anisotropic, typically ˜2 nm in the a-b plane and ˜0.2 nm along the c-axis. The coherence volume encloses very few superconducting pairs, so even the presence of small scale inhomogeneities can disrupt superconductivity. As a result, the electrical properties of Josephson junctions are sensitive to chemical changes and structural defects on atomic length scales. Consequently, to make multiple uniform HTS junctions, control at the atomic level is required.
HTS materials also exhibit highly anisotropic electrical transport. Conductivity along the c-axis is several orders of magnitude lower than that in the a-b plane. This further complicates device fabrication because the highest quality HTS films have c-axes oriented normal to the substrate. This complicates the possibility of growing epitaxial multi-layers to form sandwich-type junctions such as those created from conventional superconductors like niobium. In spite of these difficulties a number of fabrication techniques for junctions with excellent electrical properties have emerged, such as grain boundary, step-edge, and ramp-edge Josephson junctions. Unfortunately, these junctions have poor uniformity, limited scalability, and can only be made on small substrates. In addition, device fabrication is neither widely uniform nor predictable, and is more of an art-form than a manufacturable process.
Furthermore, historically, typical HTS Josephson junctions exhibit superconductor-normal metal-superconductor (SNS) properties and superconductor-insulator-superconductor (SIS) tunneling in an all HTS junction has only been observed in mechanical break junctions, some grain boundary junctions, and multi-layer c-axis sandwich junctions. These types of studies have provided a great deal of insight to theorists working on HTS. However, reproducibly fabricated tunnel junctions with well-defined interfaces, where the direction of transport can be controlled, would provide much more valuable information about the symmetry, pair wave function, and excitation spectrum. Tunneling spectroscopy has conventionally provided details about the density of excitations, the superconducting energy gap, and the coupling mechanism in conventional superconductors (electron-phonon in conventional superconductors). This information was essential to the formulation and testing of the BCS theory of superconductivity and strong coupling effects for low temperature superconductors. Some information has been obtained from the aforementioned methods, as well as from point contacts, normal metal-insulator-superconductor junctions, scanning probe microscopy, and SIS junctions between YBCO and conventional superconductors like Pb. However, a complete set of tunneling spectroscopy measurements in various symmetry directions on a length scale of the superconducting coherence length could provide more insight into the physics of the unconventional superconducting order parameters associated with HTS.
Both electron beam irradiation and masked ion implantation have long been used as methods to fabricate Josephson SNS junctions, but these junctions have suffered from very small characteristic voltages VC=ICRN (a figure of merit for Josephson junctions equal to the product of the critical current and the normal state resistance) that precludes their use in most applications. ICRN is small for these earlier SNS junctions, because the irradiated Josephson junctions are larger than the superconducting coherence length. The typical width of the trenches used in the high-aspect-ratio masks used for defining these barriers is ˜25 nm however lateral straggle of defects from the implantation process broadens out the barrier so that the actual length of the irradiated region can approach 100 nm. Josephson currents can only propagate through such large regions via the superconducting proximity effect, a phenomenon in which non-superconducting materials in close electrical proximity with a superconductor become superconducting themselves. In the case of ion irradiated Josephson junctions, the coupled materials are the same, but the irradiated region has a reduced transition temperature T′C. If the irradiated region is narrow <100 nm it will sustain a Josephson current above T′C but the pair potential Δ is significantly reduced from that of the electrodes and this results in smaller values of ICRN.
Another drawback to these ion irradiated Josephson junctions is the presence of a large non-Josephson excess current at zero voltage that does not exhibit either the DC or AC Josephson effects. The physical origin of the excess current is understood in the framework of the Blonder, Tinkham, and Klapwijk model (BTK) for microscopic electrical transport at an interface between a superconductor and a normal material. The power of this model is that it can describe current-voltage characteristics for barriers ranging from a strong barrier, such as an insulator in a tunnel junction, to a weak barrier like a normal metal, using a single parameter related to barrier strength. In the case of a strong barrier the only transport mechanism for Cooper pairs is direct Josephson tunneling whereas with weaker barriers both tunneling and Andreev reflection occur. Therefore, to maximize the Josephson current and reduce excess current a strong barrier is required but it must also be confined to less than a few nanometers wide in order for tunneling to occur as the tunneling probability depends exponentially on the insulator thickness. This dimension is too challenging for most nanofabrication techniques such as electron beam lithography or gallium focused ion beams, which were not capable of creating a narrow (˜1 nm) and strong barrier.
Accordingly, the need remains for a method for fabricating Josephson junctions and other devices with nanometer-scale dimensions. The present invention is directed to such a method.